Real-time electronic distributed control systems are an important development
of the technological evolution. Electronics are employed to control and
monitor the most safety-critical applications from flight decks to hospital
operating rooms. As these real-time systems become increasingly prevalent
and advanced, so does the demand to physically distribute the control
in strict real-time. Thus there is a need for control network protocols
that can support the application's stringent real-time requirements. Real-time
networks must provide a guarantee of service so that they will consistently
operate deterministically and correctly.
Ethernet, as defined in IEEE 802.3, is non-deterministic and thus it is
unsuitable for hard real-time applications. The media access control protocol,
CSMA/CD with its truncated binary exponential backoff algorithm, does
not allow the network to support hard real-time communication as it incorporates
random delays and allows for the possibility of transmission failure.
Decreasing costs and increasing demand for a single network type, from
boardroom to plant-floor, have led to the development of Industrial Ethernet.
The desire to incorporate a real-time element into this increasingly popular
single-network solution has led to the development of different real-time
Industrial Ethernet solutions. Fieldbus networking standards have failed
to deliver an integrated solution. Typically the emerging real-time Industrial
Ethernet solutions complement the fieldbus standards, for example by using
common user layers. This course covers an introduction to real-time systems
along with a study of Ethernet with emphasis on its suitability as a real-time
network. Module 402 provides a study of the real-time Industrial Ethernet
solutions available today.
Real-Time (RT) systems are becoming increasingly important, as industries
focus on distributed computing in automation, see Figure 1. As computing
costs decrease, and computing power increases, industry has become more
dependent on distributed computers to deliver efficiency and increased
yield to the production lines. Real-Time does not automatically mean faster
execution but rather that a process is dependent on the progression of
time for valid execution.
Figure 1 Distributed Real-Time Processing
RT systems are those whose correct execution depends not solely on the
logical validity of data but also on its timeliness. A correct RT system
will guarantee the successful operation of a system so far as its timely
execution is concerned. RT systems are generally broken into two main
sub-categories: hard and soft.
Hard Real-Time (HRT) systems are those in which incorrect operation
can lead to catastrophic events. Errors in HRT systems can cause accidents
or even death. Such systems are typically found in flight control or train
control systems, where an error could potentially incur loss of life.
Soft Real-Time (SRT) systems, on the other hand, are not as brittle.
An error in a SRT system, while not encouraged or appreciated, will not
cause loss of property or life. SRT systems are not as safety-critical
as HRT systems, and should not be employed in a safety-critical situation.
Examples of SRT systems would be online reservation systems, or streaming
multimedia applications where slight occasional delays might cause small
inconvenience but will not have serious impact.
Jobs are the real-time system's building blocks. Each real-time
job has certain temporal quantities (Figure 2) associated with them:
1. Release Time,
2. Ready Time,
3. Execution Time,
4. Response Time,
The Release Time of a job is when the job becomes available to
the system. The Execution Time is the time it takes for a job to
be completely processed. The Response Time is the interval between
the release time and the completion of the execution. The Ready Time
is the earliest time the job can start executing (always greater or equal
to the Release Time). The Deadline is the time by which execution
must be finished. If execution is not complete by the deadline, the job
is late. A job's deadline can be either hard or soft, indicating the job's
temporal dependence. As mentioned earlier, a missed hard deadline can
have serious consequences for correct system operation. All real time
systems have a certain level of jitter. Jitter is a variance on
the actual timing of the above times. In a real-time system, jitter should
be measurable within a +/ interval so that the system performance can
still be guaranteed. For textbook information on real-time systems, refer
To develop a real-time distributed system, where computers are interconnected,
it is vital to employ a network that can provide communication between
the various distributed computers in a reliable and timely fashion. Distributed
processors running real-time applications must be able to inter-communicate
via a real-time protocol, otherwise the temporal quality of their work
is lost. Real-Time Communication networks are like any real-time
system. They can be hard or soft, depending on the system requirements
and their 'jobs' include message transmission, propagation, and reception.
There are a number of real-time control networks available and employed
in industry, but none have the popularity or bandwidth capabilities of
Ethernet. In the following section, we will discuss the demand for a real-time